This invention relates to dicyclohexyl-3,4,3',4'-tetracarboxylic acid or dianhydride thereof, a process for producing the same, a polyamide-acid or polyamide-acid ester and polyimide obtained therefrom, and a process for producing the same.
Heretofore, polyimides are produced by reacting a diamine with a tetracarboxylic dianhydride in a solvent to produce a polyamide-acid, which is then subjected to dehydration and ring closure, or directly obtained by reacting a diisocyanate with a tetracarboxylic dianhydride. The thus obtained polyamide-acids and polyimides show various properties depending on the selection and combination of diamines, diisocyanates, and tetracarboxylic dianhydrides used to give, for example, those excellent in heat resistance, those rich in flexibility, those excellent in solubility, and the like.
Among them, a polyimide obtained by using an aromatic tetracarboxylic dianhydride as a tetracarboxylic dianhydride is excellent in heat resistance and mechanical properties, but is disadvantageous in that the temperature of 300.degree. C. or higher is necessary for the synthesis and the obtained polyimide has a brown appearance.
On the other hand, polyimides have been applied to various fields, for example, to optical devices, e.g. orientation films in liquid crystal display devices. When the polyimide obtained by using an aromatic tetracarboxylic dianhydride is to be applied to a liquid crystal display device, since heat resistance of the liquid crystal display device itself is poor, the device itself is decomposed during the step of polyimidization. Further, such a polyimide is low in transparency, so that the application of it to the liquid crystal display device is very difficult.
In contrast, it is known that aliphatic tetracarboxylic dianhydrides such as 1,2,3,4-butanetetracarboxylic dianhydride, or alicyclic tetracarboxylic dianhydrides such as 1,2,4,5-cyclohexyltetracarboxylic dianhydride, and 1,2,3,4-cyclopentanetetracarboxylic dianhydride can provide polyimides good in transparency by a low temperature polyimidization. But the polyimides obtained by using the aliphatic tetracarboxylic dianhydrides have a defect in that the heat resistance is very low. Further, polyimides obtained by using 1,2,4,5-cyclohexyltetracarboxylic dianhydride which is an alicyclic tetracarboxylic acid have higher glass transition temperature and pyrolysis beginning temperature than polyimides obtained by using the aliphatic tetracarboxylic dianhydride but are very brittle. On the other hand, when 1,2,3,4-cyclopentanetetracarboxylic dianhydride is used, the polymerization degree of polyamide-acid is not enhanced, and thus the resulting polyimide is very brittle.